The present invention relates generally to methods of and apparatus for measuring characteristics of airborne droplets such as size and concentration distributions, and more particularly, toward a stain type droplet sampling method and apparatus using a pneumatically operated shutter to control exposure time, wherein droplet size error caused by impact flattening is compensated and droplet deposition rate is measured as a function of stain count and shutter speed.
Knowledge of drop velocity and especially drop size concentration distribution is important in such fields as the scavenging of airborne particles by raindrops, the elimination of drop carry-over from cooling towers and dust suppression by water sprays. In a belt transfer station or other mining environment, for example, once drop size distribution, concentration and velocity are known, the spray nozzle for optimum dust suppression due to impact coverage can be selected based upon geometry, desired degree of coverage and local line pressure.
Although drop velocity for droplets of several hundred microns in diameter can be estimated in many applications, most nozzle manufacturers do not provide adequate information on droplet size and concentration to enable proper selection. Laboratory methods for measuring these parameters are available, but are complex and expensive. One form of portable droplet sampler is formed of a magnesium oxide coated, glass slide that is exposed for a predetermined time duration to airborne droplets and receives a permanent impression or stain from each droplet impacting the surface of the slide. The stains are counted and diameters measured for correlation with drop size and concentration distribution.
Because there is a "flattening effect" of a droplet in air deviating from its ideal spherical shape upon impact with a target, the stain imparted to the magnesium oxide layer will typically be larger than the actual droplet size. The effect of impact velocity of incident droplets on a target has not been previously fully understood. Drops, always heterogeneous in size, impact a solid surface at different fractions of terminal velocities, especially when the droplets fall from a limited height. Accurate measurement of droplet size by the stain technique requires a method of correcting the stain diameters to the true diameter of the incident droplet.
Engelmann, for example (see Engelmann, R. J., The Hartford Raindrop Sampler and Selected Spectra, A.E.C. Resource and Development Report, H.W. -73119, 1962), has developed a calibration curve that relates stain and drop diameters to each other. The incident drop must impact the target surface at a known velocity (usually the terminal velocity). In practice, however, physical constraints often limit the sampler location to be less than that required for the droplet to acquire terminal velocity. For example, a 0.60 millimeter diameter droplet requires a falling height of 200 centimeters to attain a fully developed terminal velocity whereas a 20 millimeter diameter droplet attains only 78% of its terminal velocity at the same height and requires 1,400 centimeters to reach its terminal velocity. Neglect of the non-terminal velocity effect for the 2.0 millimeter diameter droplet would lead to an apparent drop diameter that is 25% larger than the true diameter.
Equations have been developed that relate stain diameter to actual droplet diameter at droplet velocities that are less than terminal velocity, as set forth in Cheng, Environmental Science Technology, 11, 192 (February, 1977). In order to apply these equations to determine actual droplet size of droplets at impact velocities less than terminal velocity, it is necessary to provide a magnesium oxide layered planchet to obtain accurate stains caused by droplet impact. We have found, however, that prior art stain type droplet sampler devices are incapable of obtaining highly accurate droplet size and distribution readings.
In May, "The Measurement of Airborne Droplets by the Magnesium Oxide Method", Journal of Scientific Instruments, Volume 27, 1950, there is described a stain type droplet target using a thick magnesium oxide coating which retains stains when impacted by airborne droplets. The magnesium coating is deposited on a glass substrate at a thickness of 40-500 microns and at least 90% coverage of the target surface. We have found that the thick coating of magnesium oxide tends to acquire rough stain edges which cause inaccuracy in measurement of stain diameters. Also, due to limited dispersion of the magnesium oxide granules, the thick layer fails to distinguish whether a given stain is a result of one or more incident droplets.
Another deficiency of the prior art stain type droplet samplers has been that exposure time to ambient droplets is not accurately controlled and the exposure time is not easily varied. In Grantham, A Journal of Applied Meteorology, Volume 4, page 1293, 1975, for example, a thick magnesium oxide layer stain device is exposed to airborne droplets through a rubberband operated, rotary shutter. In practice, the shutter provides only a limited range of exposures and thus is unable to measure high or low droplet concentrations. There still exists a need, therefore, for a stain type, airborne droplet sampler that enables higher accuracy droplet size and distribution measurements than heretofore provided in the prior art.
Accordingly, one object of the present invention is to provide a new and improved airborne droplet impact sampler that provides highly accurate droplet stains when impacted by airborne droplets.
Another object of the invention is to provide a new and improved air droplet sampler that is capable of exposing an impact stain target to airborne droplets for a precisely controlled period of time.
Another object of the invention is to provide a new and improved airborne droplet sampler that is capable of exposing a droplet stain target to airborne droplets for a variable sampling period.
Another object of the invention is to provide a new and improved airborne droplet sampler stain type target that has improved stain resolution and is capable of distinguishing multiple impact stains.
Another object of the invention is to provide a new and improved airborne droplet sampler that is simple, light in weight and convenient to operate at remote locations.